This invention relates to a brake booster with a booster housing subdivided by means of a movable wall arranged therein into a chamber of constant pressure (vacuum chamber) and into a working chamber, with the movable wall being formed by a diaphragm disk and by a rolling diaphragm partially abutting the diaphragm disk, with a piston rod--coupled with a brake pedal--for actuating a control valve, which alternatively connects the working chamber with a vacuum or with a higher differential pressure wherein the control valve includes a control valve piston, axially displaceable in a control valve housing, acting on the end of a push rod through a reaction element.
A brake booster of this type has been disclosed in German Patent Specification (DE-PS) No. 29 25 550.
The particular feature about this type of brake booster is that a stop is provided at its control valve housing, which limits the initial advancing movement of the movable wall relative to the control valve housing. The stop is distanced axially at a predetermined distance with respect to the corresponding part of the movable wall when the movable wall and the control valve housing are in their respective rest positions. The movable wall is axially displaceable out of its rest position until it becomes engaged with the stop of the control valve housing, without exerting any force on the control valve housing which would carry the control valve housing along in the advancing direction. In the case of a small initial input stroke, the brake booster will have a predeterminable larger output stroke and, subsequently, an output stroke proportional to the input stroke.
The brake booster disclosed in the abovementioned patent specification also includes a power booster using a pressure source for generating a pressure difference acting on the movable wall. However, what is to be considered disadvantageous is the necessity of separating the chamber of constant pressure (vacuum chamber) from the existing vacuum source and of exposing it to the surrounding atmosphere since, in case of an excessive pressure difference, there will exist the danger of an undesired self-operation of the power booster which would result in an uncontrollable braking operation. This, however, means that there will be no boosting energy available in the case of a failure of the pressure source.